The present invention relates to magnetic recording and particularly to methods for duplicating magnetic records.
More particularly, this invention relates to methods for duplicating magnetic video records, such as may be recorded on magnetic tapes and discs.
There has existed for some time the need to make duplicate video recordings which provide high quality pictures when played back and yet are producible at low cost. The quality of the pictures depends in large measure on the strength of the signal which is recorded on the copy. The cost of the copy depends upon the complexity of the duplication technique and the time required to duplicate each copy. In the case of video magnetic records the problem is complicated by the need for recording magnetic signals on high coercivity materials (typically at least 300 to 500 Oersteds (Oe)) and at short wavelengths (typically about 1 .mu.m (micrometer). The coercivity of the tape is the ability of the tape to resist demagnetization.
One technique for duplicating video magnetic recordings is to merely play back the original magnetic signals and simultaneously re-record such signals, using a conventional magnetic head, onto a copy. The main limitation of this technique is that the rerecording takes place at the same speed as the original recording, i.e. a one hour program takes one hour to copy. The process is quite time consuming and, hence, costly.
The process of anhysteretic transfer has been examined with the view to reducing the time needed to make the copies. In anhysteretic transfer the master record and the copy are placed in face-to-face contact in the presence of an AC magnetic field. The AC field causes the magnetization pattern on the matter to transfer to the copy. In order to prevent the master record from being erased, the transfer field cannot exceed the coercive force of the master. Thus, the magnetic signal intensity on the copy is limited by the coercivity of the master. U.S. Pat. No. 3,913,131 proposes a technique for overcoming this limitation; namely, to effect anhysteretic transfer between the master and the copy at low temperatures where the coercive force of the master increases to a greater extent than the coercive force of the copy. A gain in differential coercive force of the master with respect to the copy of about 2.5 times is indicated in the patent to be obtained at low temperatures (e.g. 233 K).
In order to use the aforedescribed technique to duplicate magnetic tapes bearing slant track recordings, as is characteristic of conventional video tapes, it is necessary that the magnetic signals on the master be a mirror image of the signals which are to be anhysteretically transferred to the copy material. To produce such mirror image signals on the master, the aforementioned patent suggests using a special video recorder which records an inverted image of the signal presented to it. Such recorders tend to be costly and add significantly to the overall duplication costs. Also, like other conventional recording machines using magnetic heads, the intensity of the signals recorded by these recorders is limited because the heads used to record the signals on a magnetic material of relatively high coercivity tend to saturate at the high field levels required. Finally, the process described above requires that the master and each final copy be cooled to low temperatures so that the amplified transfer can be effected. This slows the process and, hence, increases the cost of the final copy. Thus, the problem of providing at low cost a magnetic master of very high coercivity at room temperature still remains.
There has been mentioned in the literature magnetic materials made of manganese bismuth (MnBi) particles of small dimensions (smaller than one micrometer (.mu.m)). Such materials exhibit a unique coercivity vs. temperature characteristic, viz., a coercivity which is exceptionally high, say 16,000 Oe at room temperature, and which drops dramatically to a moderately low coercivity, say 300 Oe, at 77 K, the temperature of liquid nitrogen. See, for instance, Kishimoto and Wakai, J. Appl. Phys. 48, 4640 (1977). This material has been suggested for use in a magnetic tape which is recorded at relatively low temperatures (e.g. between 170 K and 200 K) (see Japanese Patent Publication No. 134593, published on May 14, 1977).